The invention relates to solvent-spun cellulose filaments from a solution of cellulose in a tertiary amine N-oxide.
The publication "Spinning of cellulose from N-methylmorpholine-N-oxide in the presence of additives" by H. Chanzy, M. Paillet and R. Hagege (Polymer, 1990 Vol. 31, March, pages 400 to 405) discloses such filaments (fibers) which were manufactured from a solution of cellulose with a degree of polymerization (DP) of 600 or 5000 in the tertiary amine N-oxide N-methylmorpholine-N-oxide (NMMO). Based on the cellulose with a DP of 600, filaments were obtained having a strength of 0.5 GPa corresponding to 33.3 cN/tex and a 16% elongation at break. When cellulose with a DP of 5000 was employed the strength amounted to 56.7 cN/tex (0.85 GPa) and a 4% elongation at break.
By adding 2% ammonium chloride (NH.sub.4 Cl) to the cellulose solution (DP 600) the strength of the filaments produced thereby increased to 60 cN/tex (0.9 GPa). The elongation at break of these filaments amounted to 8%. Likewise by adding ammonium chloride or calcium chloride to the solutions of the cellulose with a DP of 5000, filaments with high strength were obtained (up to 87 cN/tex), but whose elongation at break was below 5%. The salts added to the solutions (NH.sub.4 Cl or CaCl.sub.2) were no longer detectable in the filaments. This effect was to be attributed to the process steps which are typical for the production of these filaments: the passage through an aqueous coagulation bath in order to precipitate the cellulose, subsequent to the spinning in an air gap and the washing baths which follow the coagulation bath. The NMMO solvent is removed from the filaments in these baths. Considering the statement in the above mentioned publication that the salts were no longer detectable in the filaments, it is to be assumed that the salts were washed out of the filaments during these treatment steps.
The solvent spinning process with a tertiary amine N-oxide as a solvent for the cellulose is characterized by its pronounced environmental acceptability since the solvent washed out of the filaments can be nearly completely recovered and be used again in preparing the solution. In respect to an economical way of carrying out the process, the addition of salts to the spinning solution is therefore disadvantageous since the recovery of the tertiary amine N-oxide, which is achieved in general with ion exchangers, becomes considerably more difficult since the resins in the ion exchangers are saturated by the ions of the salts instead of the desired degradation products of the cellulose and the amine N-oxide, which are present in the wash water in addition to the tertiary amine N-oxide. Moreover chlorides lead to corrosion of installation parts made of steel.
Besides these process-related disadvantages of adding salts to the cellulose solution, the resulting filaments also exhibit considerable disadvantages regardless of their high strength. As explained in the above mentioned publication, adding salts influences greatly the internal morphology of the filaments. In this case not only was a modification of the internal structure discovered, but also a reduction was found in the lateral cohesion compared to filaments which were produced without adding salts. Based on this fact each rubbing, bending or repetitive handling leads to a significant delamination of the filaments with a release of microfibrillar lint, which is also called fibrillation. This extremely pronounced fibrillation was not observed with filaments produced without adding salts to the spinning solution. Based on this characteristic, filaments produced under the addition of salts to the spinning solution are in general not suited for a textile application.